This invention relates to an oil balance system for compressors connected in series. More particularly, this invention relates to apparatus and a method for an oil balance system in which each compressor is contained in a separate shell, and in which each oil sump for each compressor is a low side sump, i.e., the inlet to each compressor is open to its respective shell, and the outlet from each compressor is sealed to the compressor.
My prior U.S. Pat. No. 5,839,886, the entire contents of which are incorporated herein by reference, relates to an oil balance system for primary and booster compressors connected in series for a heating/cooling or refrigeration system. The primary compressor has a low side sump, but the booster compressor has a high side sump (i.e., the inlet to the booster compressor is sealed to the compressor, and the outlet from the compressor is open to its shell. An open conduit extends between the oil sumps of the two compressors to transfer oil from the sump of the booster compressor to the sump of the primary compressor when the oil level in the booster compressor exceeds a normal operating level.
My prior U.S. Pat. Nos. 5,927,088 and 6,276,148, the entire contents of both of which are incorporated herein by reference, relate to boosted air source heat pumps especially suitable for cold weather climates. In the systems of these patents, a booster compressor and a primary compressor are connected in series.
Most compressors will entrain and pump out some oil, entrained in the refrigerant, during the normal course of operation. So, for a system of series connected compressors housed in separate casings, the pumped out oil will eventually return to the first compressor in the system, thus tending to raise the oil level in the sump of that compressor. As that oil level rises, this will likely cause the first compressor to pump oil to the inlet to the second compressor, so some oil will be delivered from that first compressor to the second compressor in the system, thus tending to prevent a dangerous loss of lubricant in the second compressor. Various compressor designs react differently in regard to this characteristic of pumping out oil entrained in the refrigerant, and it is known to make modifications to specific designs to enhance the tendency to pump out more oil as the level of oil rises.
However, during the course of operation of a series connected compressor system, such as the heat pump systems of my U.S. Pat. Nos. 5,927,088 and 6,276,148, refrigerant/oil imbalances can occur due to such things as, e.g., defrosting requirements, extreme load changes, etc. These imbalances may lead to unbalancing the oil levels in the two compressors; and this may result in taxing the normal oil balancing tendencies beyond their normal capabilities. Accordingly, it may be desirable to incorporate a specific oil balance system in the series connected compressor system.